For example in radio engineering a filter of this kind is known as a duplex filter, through which the transmitter and the receiver of an apparatus are connected to a common antenna. The filter is designed so that the transmit signal can not reach the receiver but propagates to the antenna, and so that the received signal from the antenna can not reach the transmitter but propagates to the receiver. This is possible as the filters of the transmit branch and the receive branch are dimensioned so that when one is passing, then the other is stopping. This in turn is possible only when the transmit frequency band and the receive frequency band are separated. All present duplex filters of analog radiotelephones operating in duplex mode work in this way. The same principle naturally holds true for any three-port filter, which in a common circuit means incorporates two signal paths for different frequency bands.
However, the situation is different if the three-port circuit means must process signals at the same frequency band. Then it is not possible to use the duplex principle, because the same attenuation would be present between different ports, whereby power into one port is divided into the two other ports. If the signals to be processed appear at different moments, then the three-port circuit means can be realized as a simple change-over switch being able to connect e.g. port 1 and port 2 alternately to port 3. In fact the situation is like this in radiotelephone systems operating according to the time division principle on a single frequency, and as an example of these we can mention the DECT system (Digital European Cordless Telephone). In such systems transmission and reception occur at the same frequency but in different time slots, so that a change-over switch can be used to connect the antenna to the receiver during the receive time slot and correspondingly the transmitter to the antenna during the transmit time slot.
When a change-over switch is used as a three-port circuit means for the above described purpose it must naturally be dimensioned to tolerate the highest transmit power of the apparatus, which could be of the order of 20 kW. This results in a power consuming "heavy" switch, which means attenuation of the signal passing through the switch. The attenuation of the antenna switch is of the order of 7 dB, which has an adverse effect when a weak signal is being received. An active semiconductor switch is non-linear and therefore it presents an inconvenience in circuit design. Thus a filter realized with passive components would be a good solution, but this far no three-port filter having a change-over function suitable for the processing of two signals with the same frequency was presented.
This invention presents a means using passive filter components to realize a change-over switch for signals having the same frequency but appearing at different moments. The invention is well suited to be used also when the frequencies are different, but even then time division multiplexing is required.
Here we utilize the known basic fact that the resonance frequency of a resonator can be changed, e.g. by converting a quarter-wave transmission-line resonator to be a half-wave resonator, whereby the resonance frequency is doubled. It is well known that radio frequency filters in practice may have adjusting means, e.g. adjusting screws, which in one way or another act on the capacitive load at the open end of the resonator when they are manually turned. In fact frequency shift methods are based on a similar adjustment method, which is modified so that the adjusting means is not manually but electrically controlled.
Filters based on helix resonators can use a stepping motor acting on a means moving in the capacitive or inductive field of the resonator. In a ceramic resonator a capacitance diode can be mounted at the loaded end of the resonator, between the upper end of the bore and the earthed top or side face, whereby the load capacitance and thus the resonance frequency is adjusted by adjusting the capacitance. The capacitance diode could also be mounted in the bore of the resonator.
The applicant's Finish patent application FI-913088, dated 25.6.1991, discloses a means to shift the resonator characteristic in the frequency plane. According to the method a strip line resonator is mounted in the electromagnetic field of the resonator, below called the main resonator, whereby the other end of the strip line can be shorted with a controllable switch. When the switch is open the strip line resonator is a lambda/2 resonator having the resonance frequency f.sub.O, which is so far frop the main resonator's resonance frequency that it hardly at all affects the main resonator. When the switch is closed it shorts the other end of the strip line, transforming the strip line into a lambda/4 resonator with the resonance frequency f.sub.0 /2. This is arranged to be so close to the resonance frequency of the main resonator that its resonance frequency will have a frequency shift delta f. By suitable connections at the ends of the strip line it is also possible to arrange the switch to have the opposite action compared to the above. The method is particularly well suited for application in connection with dielectric resonators, particularly with known resonator structures having on one uncoated side surface coupling patterns for the coupling to the resonator. The length of the strip line is selected to suit the resonator frequency.
The method's effect is based on the fact an electromagnetic coupling is created between the strip line and the main resonator because the strip line acts as a "secondary resonator". The stronger the coupling, the stronger effect the secondary resonator has on the frequency shift. The design of the main resonator and the dimensions of the secondary resonator and its location relative the main resonator affect the strength of the coupling. Preferably the switch can be a diode or a capacitance diode.
Some known means to shift the resonance frequency of the resonator were described above, and in this invention any known means may be used.